In recent years, sidelight-type surface light source devices have been widely used as backlight units for liquid crystal display apparatuses, and such a sidelight-type surface light source device makes light from a light source enter through a side surface (a light incident surface) of a thin plate-shaped surface light-emitting light-guiding plate, and emits diffused light through a front surface (a light-exiting surface) of the surface light-emitting light-guiding plate toward the whole area of a rear surface of a liquid crystal display element (a liquid crystal panel). However, because it is difficult to dispose a large number of light sources (e.g., LEDs) for a large amount of light so as to face the side surface of the thin plate-shaped surface light-emitting light-guiding plate which is a narrow surface, the sidelight-type surface light source device has a problem that it is difficult to improve luminance sufficiently.
To cope with this problem, there is a proposition of a surface light source device that includes a plurality of light sources (a plurality of light-emitting element arrays) arranged in a thickness direction of the surface light source device, a surface light-emitting light-guiding plate, and an optical-path changing member (e.g., a light-reflecting mirror and so on) for guiding light from the plurality of light sources to a side surface (a light incident surface) of the surface light-emitting light-guiding plate (see Patent Document 1, for example).